1. Field of the Invention
The invention relates to a control method for establishing smooth ratio changes in a multiple-ratio geared transmission for an automotive vehicle wherein the gearing elements of the transmission are characterized by a swap-shift.
2. Background Art
A conventional multiple-ratio automatic transmission for automotive vehicles includes multiple-ratio gearing having gear elements that are controlled by pressure-operated friction clutches and brakes, which are applied and released to effect ratio changes. The multiple-ratio gearing may comprise two planetary gear units, which define plural torque flow paths arranged in series disposition. One such transmission is disclosed in U.S. Pat. No. 5,835,875, which is assigned to the assignee of the present invention.
The transmission of the ""875 patent comprises two planetary gear units that can be upshifted or downshifted independently, one with respect to the other, to effect gear ratio changes. The upshifting and the downshifting of the planetary gear units occur simultaneously to accomplish an overall gear ratio upshift or downshift. This involves downshifting the first planetary gear unit by releasing an overdrive brake for a reaction element of the first planetary gear unit while simultaneously upshifting the second planetary gear unit, which has multiple ratios, by applying a brake for a reaction element of the multiple-ratio planetary gear unit. This is commonly referred to as a swap-shift.
In the control system for the ""875 patent, the release of the reaction brake for the first planetary gear unit and the application of the reaction brake for the second planetary gear unit must be accomplished synchronously. Even if a small error in the synchronization is present, the shift quality may be accompanied by a large shift shock due to inertia torque disturbances.
Swap-shift transmissions of the kind shown in the ""875 patent require precise matching of friction element application and release in order to maintain consistent shift quality. This objective is difficult to achieve, however, under all operating conditions over the life of the automotive vehicle powertrain. This is due in part to the typical changes in the braking characteristics of the friction elements as well as operating variables in the control system for the friction elements. Manufacturing tolerances for the powertrain hardware also contribute to inconsistent shift quality, as does limited data communication speed for electronic digital microcomputer components typically used for controlling the vehicle powertrain.
Swap-shift transmissions of the kind disclosed in the ""875 patent rely upon precise control of the friction elements to synchronize the behavior of two separate planetary gear units to obtain consistent shift quality. It requires an overdrive friction brake capacity reduction for the first gear unit and an intermediate ratio friction brake application for the second gear unit. The application and release of the brakes are initiated simultaneously at the beginning of the shift interval. The overdrive brake drum speed increases while the intermediate ratio brake drum speed decreases. When the intermediate brake completes its engagement, the overdrive brake capacity is synchronously brought to zero. A small output torque peak may occur as a result of the inertia torque developed during the torque transfer from the overdrive brake to an overrunning coupling in the overdrive gear unit. Shift quality becomes very sensitive to a small mismatch in timing of the overdrive brake release and the intermediate brake engagement.
In a conventional swap-shift transmission, an early release of the overdrive brake causes a sudden and large torque transfer from the overdrive brake to the overrunning coupling while the intermediate brake is still rotating. This leads to a sudden increase of input torque into the second planetary unit. As a result, intermediate brake torque capacity is substantially increased before the intermediate brake drum motion completely stops in order to complete its engagement in time. A combination of a torque transfer from the overdrive brake to the overrunning coupling and an increase in intermediate brake torque capacity results in a large output torque spike if brake application and release are not precisely timed.
A large output torque spike is perceived by the driver as a shift shock. Further, if the intermediate brake completes its engagement before the overdrive brake is fully released, a low torque capacity developed at the overdrive brake results in a sharp output torque decrease. This also is perceived by the driver as a shift shock.
The invention is a robust control method comprising a strategy to achieve consistent shift quality in a swap-shift automatic transmission of the kind disclosed in the ""875 patent. Unlike prior art control systems, the strategy of the present invention does not require precise matching of the friction element application and friction element release timing to avoid a perceptible shift shock.
The method of the invention comprises measuring the carrier speed of a first planetary gear unit, which includes an overrunning coupling for effecting a direct-drive state and a pressure-actuated reaction brake for effecting an overdrive state. The speed of the carrier of a second planetary gear unit and the speed of the sun gear for the second planetary gear unit also are measured. The overrunning speed of the overrunning coupling is controlled during a shift interval so that the reaction torque on the sun gear of the first gear unit becomes zero in a first time increment.
The speed of the sun gear of the second gear unit also is measured during a second time increment so that a second pressure-actuated reaction brake for the second gear unit becomes fully applied before the first reaction brake is fully released.
The overrunning coupling speed control provides partial torque transfer from the reaction brake for the first gear unit to the overrunning coupling before the reaction brake for the second gear unit reaches a low target speed approaching zero speed.
The control routine for the overrunning speed of the overrunning coupling occurs simultaneously with the control routine of the sun gear speed for the second gear unit. Each control routine is achieved in a closed loop fashion using actuating pressure as a variable in the control of the pressure-actuated reaction brake for the respective gear units.